Ultra-high molecular weight polyethylene is superior in abrasion resistance, self-lubrication, chemical resistance and so forth, and is applied to various applications making use of the characteristics.
By the way, it is a conventional practice, in order to cover an outer peripheral face of a roll, a pipe, a steel tube or the like to provide weathering resistance, a chemical resistance, a durability and so forth, to cover the outer peripheral face of such roll, pipe, steel tube or the like with a pipe, a film or the like. In this instance, it is a conventional practice to insert such roll, pipe, steel tube or the like into a shrink pipe or film and then make the shrink pipe or film to shrink and cover them over the roll, pipe, steel tube or the like. In such a case, shrink pipes or films made of TEFLON.RTM. are conventionally used However, TEFLON.RTM. is inferior in abrasion resistance. For example, TEFLON.RTM. has a sand abrasion loss as high as five times that of ultra-high molecular weight polyethylene.
Thus, a shrink pipe or film made of ultra-high molecular weight polyethylene has been proposed, and a method of manufacturing a shrink tube or film made of ultra-high molecular weight polyethylene, for example, by a method which has been proposed and disclosed in Japanese Patent Laid-Open Application No. 62(1987)-122736.
However, according to the method disclosed in Japanese Patent Laid-Open Application No. 62(1987)-122736, a shrink tube or film during manufacture must necessarily be sealed to exclude internal air, therefor it is folded into a flattened outer profile by means of pinch rolls. Consequently, an ultra high molecular weight polyethylene shrink pipe or film obtained has folded marks at both ear edges thereof, and if the shrink tube or film is caused to shrink to cover a roll or the like, then such folded marks will remain in the shrink tube or film. Therefore, the method cannot be adopted readily for an application for which a high degree of accuracy is required. Further, since a shrink tube obtained by the method has a thickness equal to or less than 1 mm, it is difficult to finish it by lathe machining. Besides, since such product is biaxially oriented, it is also disadvantageous in that shrinkage in a longitudinal direction is great.
It is another conventional practice that, in order to cover an outer peripheral face of a roll, a pipe, a steel tube or the like to provide a weathering resistance, a chemical resistance, a durability and so forth, an ultra-high molecular weight polyethylene thin-wall pipe is heated to 100.degree. to 130.degree. C. to expand the diameter thereof by thermal expansion and then the roll, pipe, steel tube or the like is inserted into the ultra-high molecular weight polyethylene thin pipe of the expanded diameter, whereafter the thin pipe is cooled to shrink to cover the roll, pipe, steel tube or the like.
Manufacture of the ultra-high molecular weight polyethylene thin-wall pipe is conventionally made by a method of compression molding or another method of mechanically machining a rod of ultra-high molecular weight polyethylene. However, a thin-wall pipe of an indefinite length cannot be obtained by those methods, and besides a high cost is required and it is difficult to manufacture such thin wall pipe economically.
Thus, a method of manufacturing a flexible tube made of ultra-high molecular weight polyethylene has been proposed and is disclosed in Japanese Patent Publication Application No. 2(1990)-31270 wherein ultra-high molecular weight polyethylene is melted by a screw extruder and extrusion molded from a tube die by an L/D:5 wherein a mandrel is rotated by rotation of a screw.
According to the method disclosed in Japanese Patent Publication Application No. 2(1990)-31270, flexible tubes of comparatively small diameter equal to or less then 20 mm and of comparatively thickness with a ratio of the outer diameter to the thickness equal to or less than 15 can be obtained. But the method involves draw forming, so it has drawbacks that a pipe with an inner diameter greater than an outer diameter of a mandrel used cannot be obtained and that a pipe obtained gets to expand in diameter when it is heated. Further, with the method, since an outer diameter is restricted by a sizer, it is difficult to obtain an inner surface with excellent smoothness. Consequently, it is difficult to efficiently manufacture a thin-wall pipe having a low coefficient of thermal shrinkage and having an inner surface with excellent smoothness.